The present invention relates to sensors to measure the speed and/or position of a piston in a cylinder. More particularly, the present invention relates to magnetic displacement transducers for use in vehicle suspension struts.
Various devices have been advanced to measure the distance traveled by a piston in a cylinder. One common application is determining the movement of the piston in a hydraulic, pneumatic or hydro-pneumatic vehicle suspension strut such as a shock absorber, where the piston moves axially in a cylinder filled with at least one damping medium. U.S. Pat. Nos. 4,502,006; 4,638,670 and 5,233,293 each disclose a displacement sensor to perform this function. As of yet, none of the devices so far advanced have been widely accepted in the automotive industry. Although many systems can accurately measure motion of a piston in a cylinder, a simple transducer that can be easily incorporated into the strut in order to keep manufacturing costs down is still desired by many.
As mentioned, in some prior art vehicle suspension struts, position sensors have been included for use in determining a position or a rate of movement of the piston within the cylinder. In some prior art struts having position and/or rate of movement sensors, the sensor is expensive and complicated to manufacture. Further, some prior art vehicle suspension struts have position sensors which are capable of providing information indicative of discrete (discontinuous) positions, but not information indicative of continuous positions. This limits the accuracy of the position sensor.
As an example, in U.S. Pat. No. 4,883,150 entitled xe2x80x9cHYDRAULIC SHOCK ABSORBER WITH PISTON ROD POSITION DETECTING MEANSxe2x80x9d issued on Nov. 28, 1989 to Susumu Arai, a position sensor is included in the strut to provide information indicative of the position of the piston within the cylinder. The position sensor utilizes a magnet which actuates discretely positioned switches to provide a signal representative of a detected one of multiple discrete positions of the piston within the cylinder, and thereby of a detected level of a vehicle on which the strut is installed.
Another example can be found in U.S. Pat. No. 5,233,293 entitled xe2x80x9cSENSOR FOR MEASURING THE SPEED AND/OR POSITION OF A PISTON IN RELATION TO THAT OF THE CYLINDER IT MOVES INSIDE OF IN A DASHPOT OR SHOCK ABSORBERxe2x80x9d, issued on Aug. 3, 1993 to Huang et al. This patent discloses a magnetic sensor for measuring the speed of a piston in a cylinder. The sensor is also capable of providing indications of when the piston is at the end positions of its strokes. However, the magnetic sensor disclosed in the Huang et al. patent does not provide continuous position sensing capabilities to allow for the precise detection of positions between the end positions of the piston strokes. Although lacking in these prior art struts, for precise control and/or analysis of vehicle suspension strut operation, continuous position information is required.
A vehicle suspension apparatus according to the invention includes a vehicle suspension strut and a continuous position sensor coupled to the vehicle suspension strut and adapted to provide information indicative of a continuous position of a first portion of the vehicle suspension strut relative to a second portion of the vehicle suspension strut. The continuous position sensor includes a magnet coupled to the first portion of the vehicle suspension strut and a sensing mechanism coupled to the second portion of the vehicle suspension strut and responsive to the magnet to provide an output indicative of the continuous position of the first portion relative to the second portion.